Balsa Bridge
by Garrett Boon
The Forces
Compression: Compression is a pushing (compressing) force. You can take the middle a straw and squeeze it between two fingers, the straw flattens. However, if you hold each end of a straw and push together it is harder to make the straw compress, or flatten. The shorter a piece of wood is, the more compression it can hold. The longer a piece of wood is, the less compression it can hold.
Tension: Tension is pulling. It would be hard to break a straw if you held both ends and pulled apart. Wood also has the ability to resist tension. Tension may be applied parallel to the grain of the wood, but should be avoided perpendicular to the grain. Wood is very strong in tension parallel to the grain, but extremely weak in tension perpendicular to the grain.
Torsion: Torsion is twisting. When you wring out a cloth, you are applying torsion to the cloth. If you take a stick pretzel, twist one end, and hold the other end still, it will break very easily. If you do that with a baseball bat, it will not break. However, if you take a piece of licorice and apply torsion to it, the licorice will twist around several times before it breaks. Each of these materials has a different way of responding to torsion. Bridge designers must watch for torsion and try to reduce it as much as possible.
Shear: Shear is a interesting force. It happens when there are two opposing forces acting on the same point. If you hold a piece of wood with both hands next to each other, and push up with one hand and down with the other, you are applying shear to that piece of wood. Shear usually occurs horizontally, and not vertically.
Joints
The type of joint you use on your bridge can drastically change its strength.
Lap Joint: The lap joint is one of the strongest, and you should use it whenever you can. It strengthens compression members because it adds stiffness. The lap joint has one drawback, however. The joint is only as strong as the face of the wood. That means that only one part of your piece of wood is in contact with each other. Most Balsa wood is not
End Joint: The end joint is not a very strong joint, especially for tension members. In tension, the two pieces of wood will just pull right away from each other. In compression, this joint will allow the piece to bend in a perfect arc. The lap joint holds the piece stiff, which does help it to hold more.
Notched Joint: The notched joint gives more strength than the end joint, but less than the lap joint. It is more difficult to build, so it is not very common.
Gussets: If you have to use an end joint, it is a good idea to add a gusset to make it stronger. The gusset creats a lap joint, which is strong in both tension and compression. Usually I try to make each part of the gusset the same length. If there is more glued to one piece, the one with less surface area for the glue will pull away first anyway.
Lateral Bracing: Lateral What? Exactly. Perhaps the most important aspect of your bridge and you aren’t sure what it is. Lateral bracing is the term I use to refer to any pieces on a bridge that help keep the top chord from bending horizontally.
Why is lateral bracing so important? As you may have read, the shorter a piece of wood, the more compression it can hold. Lateral bracing serves to break the top chord into smaller sections, giving it more strength. Simply put, lateral bracing is the “truss” that goes on top of your bridge.
Why only on top? The bottom member of a bridge is in tension, and is not going to bend or twist. Wood has the same tensile strength no matter how long it is. The top chord will want to bend and twist, and needs support to keep it straight.
Why an X? The reason many builders choose X’s is because they make triangles, which resist deforming. Of course, it doesn’t have to be X’s. You could use half X’s in a zigzag pattern, just take away half of the pieces in the figure. Or you could use straight pieces, circles, or any combination. However, straight pieces would just make a rectangle, which won’t really help.
There is a place and time when you can use straight pieces for lateral bracing instead of X’s. That idea is illustrated by the Fernbankbank Bridge (shown below). Notice that this bridge uses an L-beam for the top chord. And a large L-beam at that. The key lies in that fact; the straight pieces across the top had a very large surface area joining them to the top chord. This automatically made them stiff, and able to resist deforming. This kind of made a triangle, so to speak.
Bending Wood
I have developed a system for bending wood for my arch bridges, and though I do not claim it is the best way, it works well for me.
I have a pot, or basin rather, that is about 18″ long and 12″ deep. I fill it with about two cups of water (I have found that more water just takes longer to start boiling, and I usually don’t boil all of it anyway) and stick it on a stove burner. Currently I am using a stove that has a glass top and the burners are underneath, but in the past I have used one that had burners above the top of the stove.
I place any wood I am steaming on top of the basin, I usually have at least one or two pieces that are longer then the basin and some that are shorter, but the shorter ones lay on the longer pieces so that nothing is actually touching the water at this time, for the goal was to steam and not soak the wood. I turn the burner on high, and wait for the water to start boiling. I also place a shallow cookie sheet on top, to hold in the steam.
I have never steamed my wood for over a half hour, perhaps I am too impatient. But with this amount of time the wood is very bendable.
Beforehand, I have prepared two “molds” that consist of two sheets of wood, with a form of nails hammered into each, to mold the steamed wood. I have taken to making the mold a half inch or so taller then what I want the actual height of my arch, because the wood always bends a little less then the form afterwards. You may want to use more or less extra height depending on the overall height of your arch.
This form is fairly easy to make, I just set the height and length of what I want with nails, and then bend a stick of wood (usually 1/16″ by 3/16″) and hold it in place by clamping the wood to the nails with clothespins. I then add as many additional nails to the form as I see fit, along the arch of the wood.
After steaming the wood I clamp the pieces to the form and wait another thirty minutes to an hour for it to dry. Again I may be impatient, but it works for me. I then apply glue to the wood, and wait 4-5 hours for it to set (that is the amount of time my glue takes to set). I then do a little sanding, and finish building my bridge. So far, I have built an 8.5 gram bridge (as of Jan/19/04) that held 16k and broke. However, the testing apparatus was not ideal, in the fact that I forgot to bring an S hook to the place I was testing, and had to make do with something else, which I believe may have unevenly applied the load to my bridge. But then again, that may not have been the case.
I usually make my arches into the shape of an L beam, but have used a T beam in the past. I have never used an I beam, though that would most definitely be stronger but heavier. It is worth experimenting with.
Garrett Boon has spent over five years participating in Science Olympiad building and technology events. He has won over ten awards in the Bridge Building & Tower Building events, including three 1st places. In the spring of 2004, he took first place at the Georgia State Finals in Bridge Building, breaking a 4-year efficiency record with his bridge. More bridge building tips can be found on his website, Model Bridge Design.